This invention relates to a dial pulse restorer for receiving and restoring distorted telephone dial pulses and, more particularly, to such a dial pulse restorer which readily distinguishes between telephone dial pulses and a telephone hang-up pulse.
It is desirable and necessary, in various applications, to sense telephone dial pulses which may be applied to a telephone line, even though such telephone dial pulses may be distorted line, even though such telephone dial pulses may be distorted. One such application is the remote control of an operating device, such as dictation apparatus, from a remote telephone which is connected to that device via a conventional telephone line. In the environment of dictation apparatus, the various control functions of that apparatus, such as dictate, playback, rewind, fast forward, etc., conveniently can be controlled by a dictator from the remote telephone merely by dialing various predetermined numbers, each number being associated with a particular function.
In telephone systems which employ multi-frequency tones (the so-called Touch-Tone systems) for representing each dialed number, conventional tone detectors, or decoders, are provided at the site of the dictation apparatus for decoding each received Touch-Tone signal to a corresponding control signal whereby the particular function of the apparatus is performed. Generally, even if a conventional telephone line is subject to severe interference problems, these Touch-Tone signals nevertheless can be detected accurately. However, in telephone systems in which a dialed number is represented by a series of telephone dial pulses, such pulses may be significantly distorted, whereby they cannot be interpreted accurately. This disadvantage is aggravated further when a PBX system is connected between the telephone line and the dictation apparatus.
In a typical dial-pulse generator of the rotary-dial type commonly used on telephone instruments, the number of dial pulses which are generated is equal to the particular number which is dialed. Thus, one pulse is generated when a "1" is dialed, two pulses are generated when a "2" is dialed, and so on, up to ten generated pulses when a "0" is dialed. These pulses are of uniform shape, having a pulse width of 60 milliseconds and a period of 100 milliseconds, whereby these pulses are generated at the rate of 10 pulses per second. Because of distortion on the telephone line, telephone dial pulses which originate as rectangular pulses may be severely degraded such that only the positive and negative transitions of the original dial pulses are received. Such transitions are not sufficient to control the various functions of the dictation apparatus and, therefore, the original telephone dial pulses either must be reconstructed or simulated in response to the received transitions. Furthermore, it is important not to "lose" or "miss" a distorted dial pulse. For example, if six dial pulses (produced by dialing "6") controls one function and five dial pulses (produced by dialing "5") controls another function, an erroneous function will be performed if the user at a remote telephone dials "6" but, because of severe distortion, one of the six dial pulses is not detected.
In remotely controlling a device via the telephone line, it is important that the device be "disconnected" from the line when the remote telephone hangs up. In some telephone systems employing the so-called calling party control (CPC) technique, dial tone is applied to the telephone line when the calling party at the remote telephone hangs up. However, this CPC technique is not universally employed by all telephone operating companies. In some systems, a supervisory signal, such as a so-called hang-up pulse, is applied to the telephone line when the remote telephone hangs up. This hang-up pulse is known as the "click-disconnect" pulse. Although the click-disconnect pulse is not applied to the telephone line in all systems, in general, a pulse is transmitted over the telephone line when the remote telephone hangs up for practically all systems, including those which employ the CPC technique, those which employ the click-disconnect pulse technique, and others. It is important that such a hang-up pulse be discriminated from the usual dial pulses so as to avoid false operation of the dictation apparatus and, also, to assure that the dictation apparatus will be disconnected from the telephone line.
Apparatus for interpreting and decoding telephone dial pulses which are subject to severe distortion are known. One example of such apparatus is described in U.S. Pat. No. 3,444,324, issued May 13, 1969. As described in this patent, the dial pulse transition at the start of a telephone dial pulse is detected and used to generate a window pulse commencing at a time that is delayed by 30 milliseconds from such transition and which is centered upon the expected time of occurrence of the next following transition which represents the completion of the telephone dial pulse. If this second transition occurs during the window pulse, a flip-flop circuit is actuated, thus representing the receipt of a telephone dial pulse. If the window pulse is produced in response to a spurious telephone pulse, the flip-flop circuit is not actuated. While this patent describes a telephone dial pulse detector that can discriminate between dial pulses and spurious pulses, it is not capable of detecting or discriminating a hang-up pulse.
Other telephone dial pulse detecting apparatus proceeds under different principles of operation than that followed in the aforedescribed patent. For example, in U.S. Pat. No. 3,927,264, issued Dec. 16, 1975, telephone dial pulses are sensed on the basis of the amplitude of each received pulse. Initially, that is, prior to effecting control over a device from a remote telephone, a train of dial pulses is generated, such as by dialing "9". The pulses in this train are compared to a set of threshold levels. The highest threshold level which is exceeded by these pulses is used as a reference to detect whether subsequent pulses are true telephone dial pulses or merely spurious noise. The apparatus described in this patent also senses whether the spacing between successive pulses exceeds the standard for telephone dial pulses. Unfortunately, this apparatus is susceptible either to false triggering or to "missed" pulses in the event that conditions on the telephone line vary from the initial conditions that were present when the initial train of dial pulses was generated.
Another example of dial pulse detecting apparatus is described in U.S. Pat. No. 4,006,316, issued Feb. 1, 1977. In this patent, telephone dial pulses are detected on the basis of their frequency and timing intervals. Such a system is relatively complex and expensive to construct.
Yet another example of dial pulse detecting apparatus is described in Volume 18, No. 4, of the IBM Technical Disclosure Bulletin, September 1975, in the article entitled "Dial Pulse Detector" by Bringol et al. (pages 991-992), in which incoming pulses, both telephone dial pulses and spurious noise pulses, are counted. Depending upon the correlation between the number of counted pulses and an expected number, a determination is made as to whether the received pulses represent a dialed digit or merely are noise pulses. This apparatus also is relatively complex and expensive.
The use of dial pulse detecting apparatus in the environment of a dictation system is suggested, broadly, in Volume 14, No. 6 of the IBM Technical Disclosure Bulletin, dated Nov. 1971, in the article entitled "Control Device for the Operation of Dictation Units" by Diehl et al. (pages 1701-1702). This article, however, does not disclose the type of dial pulse detecting apparatus which is used.
In view of the aforenoted requirements of dial pulse detecting apparatus, and particularly, the ability of such apparatus to discriminate between telephone dial pulses which are distorted and hang-up pulses which also may be distorted, there is a need for improved dial pulse detecting apparatus, and particularly such apparatus which can be used to control the various functions of a dictation system.